1. Field of the Invention
The invention relates to wellbore completion operations. More specifically, the present invention relates to an apparatus and method for isolating wellbore pressure during tool removal.
2. Description of Related Art
Certain devices known as downhole tools 12 are inserted into a wellbore 5 for various reasons relating to exploration, completion, and production of a wellbore 5. These tools include imaging devices, retrieval tools, and perforating guns, to name but a few. As is known, these tools are often inserted into the wellbore 5 under pressure. That is the pressure within the wellbore 5 might far exceed the ambient pressure at the surface. Thus, for safety concerns, the differential between the wellbore pressure and the surface pressure must be maintained when inserting or removing downhole tools 12 from the wellbore 5. In order to maintain this pressure differential between pressurized wellbores and the surface, devices known as “lubricators” are often employed to seal around the inserted tool and prevent pressure leakage from the wellbore.
A lubricator is typically comprised of one or more tubular members that form a sealed chamber around a downhole tool. The lubricator is usually attached to a pressure containment spool, such as a valve or blowout preventer at the top of the wellhead. At an upper end of the lubricator, sealing equipment such as a grease injector and/or a stuffing box seals the top of the lubricator, while permitting the downhole tool to be suspended by a downhole tool insertion string, a wireline for example, that extends through the sealing equipment. Thus, a sealed chamber is provided within the lubricator above a closure mechanism of the pressure containment spool e.g. blow out preventer (BOP) or a Christmas Tree. The sealed chamber houses the downhole tool and contains well pressure while the downhole tool is inserted into the wellbore. Pressure between the wellbore and the lubricator is equalized using an equalizer valve or other means by which the pressure above the pressure barrier (e.g. the BOP or Christmas Tree) can be equalized to that below. The closure mechanism of the pressure containment spool is then opened, allowing access to the wellbore. The downhole tool 12 is lowered into the wellbore by manipulating the downhole tool insertion string.
In many instances however, the length of the downhole tool 12 far exceeds that of currently available lubricators. Optionally a valve can be situated within the wellbore 5 to act as a means by which the surface can be isolated from the pressure in the wellbore below the valve. Once the valve is shut, the region above the isolation is bled to atmosphere and the tool 12 is removed from within the wellbore 5. If the isolation valve is not operating properly, this can expose the surface personnel to the possible dangers of full wellbore pressure. Therefore, there exists a need for a method and device capable of safely isolating wellbore pressure from surface pressure that can perform this function during deployment and retrieval of downhole tools.